Once a borehole has been drilled into the earth for the purpose of penetrating earth formations, pipe strings are run into the borehole in order to facilitate the flow of earth fluids to the surface. Such pipe strings are assembled from pipe string sections, typically having a pin at one end and a box member at the other end, and the pipe sections are run into the borehole as the sections are interconnected.
The interconnection or assembly of the pipe string sections is carried out by the use of a power tong to connect the pin of one section to the box of another. The power tong typically comprises a rotary jaw member for gripping the pipe and a motor for rotating the jaw member until the pipe string sections have been tightened to a desired degree. The joint must be tight enough to prevent leakage while developing high joint strength, but the joint must not be so tight as to damage the threads of either the box member or the pin of the connected pipe sections.
In the past, a number of techniques have been used to determine the amount of torque required to connect the pipe string sections together with the desired degree of tightness. Some techniques have involved measurement of both the torque and the number of revolutions of the pipe string sections by the power tong. For example, see the following U.S. Pat. Nos.: 3,368,396; 3,606,664; 3,745,820; 4,068,189; 4,091,451; 4,176,436; 4,210,017; and 4,365,402.
Other disclosures of general background interest relative to the invention are contained in the following U.S. Pat. Nos.: 1,907,461; 1,985,916; 2,527,456; 2,971,373; 3,390,575; 3,492,866; 3,527,094; and 4,738,145. In addition, the following publication is of general background interest: "Portable Roundness-Tester for Checking Bores in Large Components", Machines and Tooling, Volume XLI, No. 1 (1970), pp. 57-58.
Some arrangements and techniques, such as that disclosed in U.S. Pat. No. 4,738,145, employ a torque-only plot in order to interconnect pipe sections. However, such arrangements and technique of the prior art produce inaccurate results under certain conditions, such as when: (1) defects are present in the pipe connections (such defects can be due to damage during manufacture, use, storage or handling; (2) the tubing (typically 30 feet or longer) is bent; (3) the tubing extends for such a length (three sections or ninety feet) so as to cause the threads to carry extra weight and possibly wobble during assembly; (4) wind or derrick misalignment causes pipe misalignment, resulting in inaccurate torque plots; or (5) too much lubricant, too little lubricant or the wrong type of lubricant adversely affecting the torque plot.
In some arrangements of the prior art, strain gauges have been used in many forms and applications. However, other devices such as LVDT devices and the inductance family of linear measuring devices are considered easier to use, depending on the skill of the user. Nevertheless, in light of the devices of the prior art, it is considered desirable to develop a measuring device which provides a strain plot without dimensions and settings being critical to good test results, and which is portable, does not require a great deal of skill to use, does not require that a strain gauge be attached directly to the surface being tested, is easy to calibrate, is relatively inexpensive, and is available in an intrinsically safe embodiment.
Prior to describing the invention, a few definitions should be set forth. The term "stress" is a term used to compare the force applied to a material and the ability of the material to carry the force. Stress cannot be measured directly, and it must be computed from other measurable parameters, one of which is strain.
With respect to the term "strain" when a force is applied to a material and the material deforms, this deformation is called "strain". Strain can be given a value by measuring the deformation of a certain unit of length of the material before a force is applied, and then measuring the deformation of the unit of length after a force is applied. Strain can be either tensile or compressive. Moreover, strain can be measured by various mechanical and electrical methods.
The term "torque" is a unit that measures rotational motion around an axis. It is the product of a force and the moment arm of the force, the moment arm being the perpendicular distance from the axis to the line of action of the force.
Finally, the terms "torsion" or "torsional" refer to the twisting or wrenching of a body by the exertion of forces tending to turn one end or part of the body about a longitudinal axis while the other end is held fast or turned in the opposite direction.